The present invention relates to a method of compensating for the failure of a dot generating unit in a printing system including a multiple-unit printhead scanning an image receiving medium in line direction and capable of printing several lines at a time, wherein multi-pass printing and interleaved line feed are employed and wherein as a unit fails the image information associated with that unit for each pass of the printhead is printed with at least one other unit of the printhead during at least one of the other passes.
A multiple-unit printhead comprises a plurality of dot generating units arranged in an array which extends perpendicular to the direction of the printing lines on the image receiving medium. Thus, image dots can be printed simultaneously in a plurality of lines, while the printhead performs a single scanning stroke or pass across the image receiving medium. For example, in the case of an ink jet printer, each dot generating unit is formed by a single nozzle and an associated actuator system by which ink droplets are jetted out from the nozzle in response to a drop demand signal supplied thereto in accordance with the image information to be printed.
Multi-pass printing means that only part of the image information is printed in a single pass of the printhead, i.e. during the movement of the printhead in a unique direction over the entire length of the line, and the printed line is completed in one or more subsequent passes. For example, in case of a two-pass system, every second dot or pixel is printed during the forward pass of the printhead and the missing dots are inserted during the second pass.
Interleaved line feed means that at least two different units or nozzles of the printhead contribute to the printing of each image line. This is achieved by feeding the image recording medium in a direction normal to the image lines in steps that have a width smaller than the extension of the nozzle array in that direction, so that the nozzle array sweeps at least twice over each location on the receiving medium.
An example of a printing system with the above features is disclosed in US-A-5,359,355. When, in this system, one of the dot generating units of the printhead becomes inoperative, e.g. because the nozzle has become clogged or air is trapped in the actuator system, the image information can not be printed completely. If, for example, the printhead has eight nozzles and one of them is inoperative, then, in a single-pass system or a system in which no interleaved line feed is employed, every eighth line will be missing on the printed document. In the case of a two-pass system with interleaved line feed, every second dot or pixel will be missing in every fourth line.
JP-A-60-104 335 discloses an ink jet printer, in which additional nozzles are provided in reserve on the printhead. If one of the regular nozzles fails, the printing pattern is changed, so that one or more of the reserve nozzles are activated in order to compensate for the failure. In this system, however, the number of usable nozzles is always limited to the maximal number of consecutive nozzles in the array that are operative. As a result, a certain loss of production of the printer must generally be expected, depending on the location where the nozzle failure occurs.